Voltage threshold ablation apparatus

ABSTRACT

A method and apparatus for treating tissue using an electrosurgical system. The system includes an electrosurgical system having an RF generator, a treatment electrode electrically coupled to the RF generator and positioned in contact with target tissue to be treated, and a spark gap switch positioned between the RF generator and the target tissue. The spark gap includes a threshold voltage and is configured to prevent conduction of current from the RF generator to the tissue until the voltage across the spark gap reaches the threshold voltage.  
     The method includes the steps of using the RF generator to apply a voltage across the spark gap switch, the spark gap switch causing conduction of current from the RF generator to the target tissue once the voltage across the spark gap reaches the threshold voltage.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This is a continuation of U.S. patent application Ser. No.09/631,040 filed on 1 Aug. 2000 and entitled Voltage Threshold AblationMethod and Apparatus.

FIELD OF THE INVENTION

[0002] The present invention relates to the field of electrosurgery, andmore particularly to methods for ablating, cauterizing and/orcoagulating body tissue using radio frequency energy.

BACKGROUND OF THE INVENTION

[0003] Radio frequency ablation is a method by which body tissue isdestroyed by passing radio frequency current into the tissue. Some RFablation procedures rely on application of high currents and lowvoltages to the body tissue, resulting in resistive heating of thetissue which ultimately destroys the tissue. These techniques sufferfrom the drawback that the heat generated at the tissue can penetratedeeply, making the depth of ablation difficult to predict and control.This procedure is thus disadvantageous in applications in which only afine layer of tissue is to be ablated, or in areas of the body such asthe heart or near the spinal cord where resistive heating can result inundesirable collateral damage to critical tissues and/or organs.

[0004] It is thus desirable to ablate such sensitive areas using highvoltages and low currents, thus minimizing the amount of current appliedto body tissue.

SUMMARY OF THE INVENTION

[0005] The present invention is a method and apparatus for treatingtissue using an electrosurgical system. The system includes anelectrosurgical system having an RF generator, a treatment electrodeelectrically coupled to the RF generator and positioned in contact withtarget tissue lo be treated, and a spark gap switch positioned betweenthe RF generator and the target tissue. The spark gap includes athreshold voltage and is configured to prevent conduction of currentfrom the RF generator to the tissue until the voltage across the sparkgap reaches the threshold voltage.

[0006] A method according to the present invention includes the steps ofusing the RF generator to apply a voltage across the spark gap switch,the spark gap switch causing conduction of current from the RF generatorto the target tissue once the voltage across the spark gap reaches thethreshold voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007]FIG. 1 is a cross-sectional side elevation view of a firstembodiment of an ablation device utilizing principles of the presentinvention.

[0008]FIG. 2 is an end view showing the distal end of the device of FIG.1.

[0009]FIG. 3 is a graphical representation of voltage output from an RFgenerator output over time.

[0010]FIG. 4A is a graphical representation of voltage potential acrossa body tissue load, from an ablation device utilizing voltage thresholdablation techniques as described herein.

[0011]FIG. 4B is a graphical representation of voltage potential acrossa body tissue load, from an ablation device utilizing voltage thresholdablation techniques as described herein and further utilizing techniquesdescribed herein for decreasing the slope of the trailing edge of thewaveform.

[0012]FIGS. 5A through 5D are a series of cross-sectional side elevationviews of the ablation device of FIG. 1, schematically illustrating useof the device to ablate tissue.

[0013]FIG. 6A is a cross-sectional side view of a second embodiment ofan ablation device utilizing principles of the present invention.

[0014]FIG. 6B is an end view showing the distal end of the device of thedevice of FIG. 6A.

[0015]FIGS. 7A and 7B are cross-sectional side elevation view of a thirdembodiment of an ablation device utilizing principles of the presentinvention. In FIG. 7A, the device is shown in a contracted position andin FIG. 7B the device is shown in an expanded position.

[0016]FIG. 8A is a perspective view of a fourth embodiment of anablation device utilizing principles of the present invention.

[0017]FIG. 8B is a cross-sectional side elevation view of the ablationdevice of FIG. 8A.

[0018]FIG. 9A is a perspective view of a fifth embodiment of an ablationdevice utilizing principles of the present invention.

[0019]FIG. 9B is a cross-sectional side elevation view of the ablationdevice of FIG. 9A.

[0020]FIG. 10 is a cross-sectional side elevation view of a sixthablation device utilizing principles of the present invention.

[0021]FIG. 11A is a perspective view of a seventh embodiment of anablation device utilizing principles of the present invention.

[0022]FIG. 11B is a cross-sectional side elevation view of the ablationdevice of FIG. 11A.

[0023]FIG. 11C is a cross-sectional end view of the ablation device ofFIG. 11A.

[0024]FIG. 12A is a perspective view of an eighth embodiment of anablation device utilizing principles of the present invention.

[0025]FIG. 12B is a cross-sectional side elevation view of the ablationdevice of FIG. 12A.

[0026]FIG. 13A is a cross-sectional side elevation view of a ninthembodiment of an ablation device utilizing principles of the presentinvention.

[0027]FIG. 13B is a cross-sectional end view of the ablation device ofFIG. 13A, taken along the plane designated 13B-13B in FIG. 13A.

[0028]FIG. 14A is a cross-sectional side elevation view of a tenthembodiment of an ablation device utilizing principles of the presentinvention.

[0029]FIG. 14B is a front end view of the grid utilized in theembodiment of FIG. 14A.

[0030]FIG. 15A is a cross-sectional side elevation view of an eleventhembodiment.

[0031]FIG. 15B is a cross-sectional end view of the eleventh embodimenttaken along the plane designated 15B-15B in FIG. 15A.

[0032]FIG. 15C is a schematic illustration of a variation of theeleventh embodiment, in which the mixture of gases used in the reservoirmay be adjusted so as to change the threshold voltage.

[0033] FIGS. 16A-16D are a series of drawings illustrating use of theeleventh embodiment.

[0034]FIG. 17 is a series of plots graphically illustrating the impactof argon flow on the ablation device output at the body tissue/fluidload.

[0035]FIG. 18 is a series of plots graphically illustrating the impactof electrode spacing on the ablation device output at the bodytissue/fluid load.

[0036]FIG. 19 is a schematic illustration of a twelfth embodiment of asystem utilizing principles of the present invention, in which a sparkgap spacing may be selected so as to pre-select a threshold voltage.

DETAILED DESCRIPTION

[0037] Several embodiments of ablation systems useful for practicing avoltage threshold ablation method utilizing principles of the presentinvention are shown in the drawings. Generally speaking, each of thesesystems utilizes a switching means that prevents current flow into bodytissue until the voltage across the switching means reaches apredetermined threshold potential. By preventing current flow intotissue until a high threshold voltage is reached, the inventionminimizes collateral tissue damage that can occur when a large amount ofcurrent is applied to the tissue. The switching means may take a varietyof forms, including but not limited to an encapsulated or circulatedvolume of argon or other fluid/gas that will only conduct ablationenergy from an intermediate electrode to the ablation electrodes once ithas been transformed to a plasma by being raised to a threshold voltage.

[0038] The embodiments described herein utilize a spark gap switch forpreventing conduction of energy to the tissue until the voltagepotential applied by the RF generator reaches a threshold voltage. In apreferred form of the apparatus, the spark gap switch includes a volumeof fluid/gas to conduct ablation energy across the spark gap, typicallyfrom an intermediate electrode to an ablation electrode. The fluid/gasused for this purpose is one that will not conduct until it has beentransformed to conductive plasma by having been raised to a thresholdvoltage. The threshold voltage of the fluid/gas will vary withvariations in a number of conditions, including fluid/gas pressure,distance across the spark gap (e.g. between an electrode on one side ofthe spark gap and an electrode on the other side of the spark gap), andwith the rate at which the fluid/gas flows within the spark gap—ifflowing fluid/gas is used. As will be seen in some of the embodiments,the threshold voltage may be adjusted in some embodiments by changingany or all of these conditions.

[0039] A first embodiment of an ablation device 10 utilizing principlesof the present invention is shown in FIGS. 1-2. Device 10 includes ahousing 12 formed of an insulating material such as glass, ceramic,siliciumoxid, PTFE or other material having a high melting temperature.At the distal end 13 of the housing 12 is a sealed reservoir 20. Aninternal electrode 22 is disposed within the sealed reservoir 20.Electrode 22 is electrically coupled to a conductor 24 that extendsthrough the housing body. Conductor 24 is coupled to an RF generator 28which may be a conventional RF generator used for medical ablation, suchas the Model Force 2 RF Generator manufactured by Valley Lab. A returnelectrode 30 is disposed on the exterior surface of the housing 12 andis also electrically coupled to RF generator 28.

[0040] A plurality of ablation electrodes 32 a-c are located on thedistal end of the housing 12. Ablation electrodes 32 a-c may be formedof tungsten or any conductive material which performs well when exposedto high temperatures. In an alternative embodiment, there may be onlyone ablation electrode 32, or a different electrode configuration.

[0041]FIGS. 5A through 5D illustrate the method of using the embodimentof FIG. 1. Referring to FIGS. 5A, prior to use the reservoir 20 isfilled with a fluid or gas. Preferably, an inert gas such as argon gasor a similar gas such as Neon, Xenon, or Helium is utilized to preventcorrosion of the electrodes, although other fluids/gases could beutilized so long as the electrodes and other components wereappropriately protected from corrosion. For convenience only, theembodiments utilizing such a fluid/gas will be described as being usedwith the preferred gas, which is argon.

[0042] It should be noted that while the method of FIGS. 5A-5D is mostpreferably practiced with a sealed volume of gas within the reservoir20, a circulating flow of gas using a system of lumens in the housingbody may alternatively be used. A system utilizing a circulating gasflow is described in connection with FIGS. 15A-15B.

[0043] The distal end of the device 10 is placed against body tissue tobe ablated, such that some of the electrodes 32 a, 32 b contact thetissue T. In most instances, others of the electrodes 32 c are disposedwithin body fluids B. The RF generator 28 is powered on and graduallybuildsup the voltage potential between electrode 22 and electrodes 33a-33 c.

[0044] Despite the voltage potential between the internal electrode 22and ablation electrodes 32 a-c, there initially is no conduction ofcurrent between them. This is because the argon gas will not conductcurrent when it is in a gas phase. In order to conduct, high voltagesmust be applied through the argon gas to create a spark to ionize theargon and bring it into the conductive plasma phase. Later in thisdescription these voltages may also be referred to as “initiatingvoltages” since they are the voltages at which conduction is initiated.

[0045] The threshold voltage at which the argon will begin toimmediately conduct is dependent on the pressure of the argon gas andthe distance between electrode 22 and surface electrodes 32 a-32 c.

[0046] Assume P1 is the initial pressure of the argon gas withinreservoir 20. If, at pressure P1, a voltage of V1 is required to igniteplasma within the argon gas, then a voltage of V>V1 must be applied toelectrode 22 to ignite the plasma and to thus begin conduction ofcurrent from electrode 22 to ablation electrodes 32 a-32 c.

[0047] Thus, no conduction to electrodes 32 a-32 c (and thus into thetissue) will occur until the voltage potential between electrode 22 andablation electrodes 32 a-32 c reaches voltage V. Since no current flowsinto the tissue during the time when the RF generator is increasing itsoutput voltage towards the voltage threshold, there is minimal resistiveheating of the electrodes 32 a-32 c and body tissue. Thus, this methodrelies on the threshold voltage of the argon (i.e. the voltage at whicha plasma is ignited) to prevent overheating of the ablation electrodes32 a, 32 b and to thus prevent tissue from sticking to the electrodes.

[0048] The voltage applied by the RF generator to electrode 22 cyclesbetween +V and −V throughout the ablation procedure. However, as theprocess continues, the temperature within the reservoir begins toincrease, causing the pressure of the argon to likewise increase. As thegas pressure increases, the voltage needed to ignite the plasma alsoincreases. Eventually, increases in temperature and thus pressure willcause the voltage threshold needed to ignite the plasma to increaseabove V. When this occurs, flow of current to the ablation electrodeswill stop until the argon temperature and pressure decrease to a pointwhere the voltage required for plasma ignition is at or below V. Initialgas pressure P1 and the voltage V are thus selected such that currentflow will terminate when the electrode temperature is reaching a pointat which tissue will stick to the electrodes.

[0049] The effect of utilizing a minimum voltage limit on the potentialapplied to the tissue is illustrated graphically in FIGS. 3 and 4A. FIG.3 shows RF generator voltage output V_(RF) over time, and FIG. 4A showsthe ablation potential V_(A) between internal electrode 22 and bodytissue. As can be seen, V_(A) remains at 0V until the RF generatoroutput V_(RF) reaches the device's voltage threshold V_(T), at whichtime V_(A) rises immediately to the threshold voltage level. Ablationvoltage V_(A) remains approximately equivalent to the RF generatoroutput until the RF generator output reaches 0V. V_(A) remains at 0Vuntil the negative half-cycle of the RF generator output falls below(−V_(T)), at which time the potential between electrode 22 and thetissue drops immediately to (−V_(T)), and so on. Because there is noconduction to the tissue during the time that the RF generator output isapproaching the voltage threshold, there is little conduction to thetissue during low voltage (and high current) phases of the RF generatoroutput. This minimizes collateral tissue damages that would otherwise becaused by resistive heating.

[0050] It is further desirable to eliminate the sinusoidal trailing endof the waveform as an additional means of preventing application of lowvoltage/high current to the tissue and thus eliminating collateraltissue damage. Additional features are described below with respectFIGS. 14A-18. These additional features allow this trailing edge to beclipped and thus produce a waveform measured at the electrode/tissueinterface approximating that shown in FIG. 4B.

[0051] Another phenomenon occurs between the electrodes 32 a-32 c andthe tissue, which further helps to keep the electrodes sufficiently coolas to avoid sticking. This phenomenon is best described with referenceto FIGS. 5A through 5D. As mentioned, in most cases some of theelectrodes such as electrode 32 c will be in contact with body fluidwhile others (e.g. 32 a-b) are in contact with tissue. Since theimpedance of body fluid F is low relative to the impedance of tissue T,current will initially flow through the plasma to electrode 32 c andinto the body fluid to return electrode 30, rather than flowing to theelectrodes 32, 32 b that contact tissue T.

[0052] Resistive heating of electrode 32 c causes the temperature ofbody fluid F to increase. Eventually, the body fluid F reaches a boilingphase and a resistive gas/steam bubble G will form at electrode 32 c.Steam bubble G increases the distance between electrode 22 and bodyfluid F from distance D1 to distance D2 as shown in FIG. 5B. The voltageat which the argon will sustain conductive plasma is dependent in parton the distance between electrode 22 and the body fluid F. If thepotential between electrode 22 and body fluid F is sufficient tomaintain a plasma in the argon even after the bubble G has expanded,energy will continue to conduct through the argon to electrode 32 c, andsparking will occur through bubble G between electrode 32 c and the bodyfluid F.

[0053] Continued heating of body fluid F causes gas/steam bubble G tofurther expand. Eventually the size of bubble G is large enough toincrease the distance between electrode 22 and fluid F to be greatenough that the potential between them is insufficient to sustain theplasma and to continue the sparking across the bubble G. Thus, theplasma between electrodes 22 and 32 c dies, causing sparking todiscontinue and causing the current to divert to electrodes 32 a, 32 binto body tissue T, causing ablation to occur. See FIG. 5C. A gas/steaminsulating layer L will form in the region surrounding the electrodes 32a, 32 b. By this time, gas/steam bubble G around electrode 32 c may havedissipated, and the high resistance of the layer L will cause thecurrent to divert once again into body fluid B via electrode 32 c ratherthan through electrodes 32 a, 32 b. This process may repeat many timesduring the ablation procedure.

[0054] A second embodiment of an ablation device 110 is shown in FIGS.6A and 6B. The second embodiment operates in a manner similar to themethod described with respect to the first embodiment, but it includesstructural features that allow the threshold voltage of the argon to bepre-selected. Certain body tissues require higher voltages in order forablation to be achieved. This embodiment allows the user to select thedesired ablation voltage and to have the system prevent currentconduction until the pre-selected voltages are reached. Thus, there isno passage of current to the tissue until the desired ablation voltageis reached, and so there is no unnecessary resistive tissue heatingduring the rise-time of the voltage.

[0055] As discussed previously, the voltage threshold of the argonvaries with the argon pressure in reservoir 120 and with the distance dacross the spark gap, i.e. the distance extending between electrode 122and ablation electrodes 132 a-c. The second embodiment allows the argonpressure and/or the distance d to be varied so as to allow the voltagethreshold of the argon to be pre-selected to be equivalent to thedesired ablation voltage for the target tissue. In other words, if atreatment voltage of 200V is desired, the user can configure the secondembodiment such that that voltage will be the threshold voltage for theargon. Treatment voltages in the range of 50V to 10,000V, and mostpreferably 200V-500V, may be utilized.

[0056] Referring to FIG. 6A, device 110 includes a housing 112 formed ofan insulating material such as glass, ceramic, siliciumoxid, PTFE orother high melting temperature material. A reservoir 120 housing avolume of argon gas is located in the housing's distal tip. A plunger121 is disposed within the housing 112 and includes a wall 123. Theplunger is moveable to move the wall proximally and distally betweenpositions 121A and 121B to change the volume of reservoir 120. Plungerwall 123 is sealable against the interior wall of housing 112 so as toprevent leakage of the argon gas.

[0057] An elongate rod 126 extends through an opening (not shown) inplunger wall 123 and is fixed to the wall 123 such that the rod and wallcan move as a single component. Rod 126 extends to the proximal end ofthe device 110 and thus may serve as the handle used to move the plunger121 during use.

[0058] Internal electrode 122 is positioned within the reservoir 120 andis mounted to the distal end of rod 126 such that movement of theplunger 121 results in corresponding movement of the electrode 122.Electrode 122 is electrically coupled to a conductor 124 that extendsthrough rod 126 and that is electrically coupled to RF generator 128.Rod 126 preferably serves as the insulator for conductor 124 and as suchshould be formed of an insulating material.

[0059] A return electrode 130 is disposed on the exterior surface of thehousing 112 and is also electrically coupled to RF generator 128. Aplurality of ablation electrodes 132 a, 132 b etc. are positioned on thedistal end of the housing 112.

[0060] Operation of the embodiment of FIGS. 6A-6B is similar to thatdescribed with respect to FIGS. 5A-5B, and so most of that descriptionwill not be repeated. Operation differs in that use of the secondembodiment includes the preliminary step of moving rod 126 proximally ordistally to place plunger wall 123 and electrode 122 into positions thatwill yield a desired voltage threshold for the argon gas. Moving theplunger in a distal direction (towards the electrodes 132 a-c) willdecrease the volume of the reservoir and accordingly will increase thepressure of the argon within the reservoir and vice versa. Increases inargon pressure result in increased voltage threshold, while decreases inargon pressure result in decreased voltage threshold.

[0061] Moving the plunger 126 will also increase or decrease thedistance d between electrode 122 and electrodes 132 a-c. Increases inthe distance d increase the voltage threshold and vice versa.

[0062] The rod 126 preferably is marked with calibrations showing thevoltage threshold that would be established using each position of theplunger. This will allow the user to move the rod 126 inwardly (toincrease argon pressure but decrease distance d) or outwardly (todecrease argon pressure but increase distance d) to a position that willgive a threshold voltage corresponding to the voltage desired to beapplied to the tissue to be ablated. Because the argon will not igniteinto a plasma until the threshold voltage is reached, current will notflow to the electrodes 132 a, 132 b etc. until the pre-selectedthreshold voltage is reached. Thus, there is no unnecessary resistivetissue heating during the rise-time of the voltage.

[0063] Alternatively, the FIG. 6A embodiment may be configured such thatplunger 121 and rod 126 may be moved independently of one another, sothat argon pressure and the distance d may be adjusted independently ofone another. Thus, if an increase in voltage threshold is desired,plunger wall 123 may be moved distally to increase argon pressure, orrod 126 may be moved proximally to increase the separation distancebetween electrode 122 and 132 a-c. Likewise, a decrease in voltagethreshold may be achieved by moving plunger wall 123 proximally todecrease argon pressure, or by moving rod 126 distally to decrease theseparation distance d. If such a modification to the FIG. 6A wasemployed, a separate actuator would be attached to plunger 121 to allowthe user to move the wall 123, and the plunger 126 would be slidablerelative to the opening in the wall 123 through which it extends.

[0064] During use of the embodiment of FIGS. 6A and 6B, it may bedesirable to maintain a constant argon pressure despite increases intemperature. As discussed in connection with the method of FIGS. 5A-5D,eventual increases in temperature and pressure cause the voltage neededto ignite the argon to increase above the voltage being applied by theRF generator, resulting in termination of conduction of the electrodes.In the FIG. 6A embodiment, the pressure of the argon can be maintaineddespite increases in temperature by withdrawing plunger 121 gradually asthe argon temperature increases. By maintaining the argon pressure, thethreshold voltage of the argon is also maintained, and so argon plasmawill continue to conduct current to the electrodes 132 a 132 b etc.Similarly, the position of electrode 122 may be changed during use so asto maintain a constant voltage threshold despite argon temperatureincreases.

[0065]FIGS. 7A and 7B show an alternative embodiment of an ablationdevice 210 that is similar to the device of FIGS. 6A and 6B. In thisembodiment, argon is sealed within reservoir 220 by a wall 217. Ratherthan utilizing a plunger (such as plunger 121 in FIG. 6A) to change thevolume of reservoir 220, the FIGS. 7A-7B embodiment utilizes bellows 221formed into the sidewalls of housing 212 and a pullwire 226 (which maydouble as the insulation for conductor 224) extending through internalelectrode 222 and anchored to the distal end of the housing 212. Pullingthe pullwire 226 collapses the bellows into a contracted position asshown in FIG. 7A and increases the pressure of the argon within thereservoir 220. Advancing the pullwire 226 expands the bellows as shownin FIG. 7B, thereby decreasing the pressure of the argon. The pullwireand bellows may be used to pre-select the threshold voltage, since (fora given temperature) increasing the argon pressure increases thethreshold voltage of the argon and vice versa. Once the thresholdvoltage has been pre-set, operation is similar to that of the previousembodiments. It should be noted that in the third embodiment, thedistance between electrode 222 and ablation electrodes 232 a-c remainsfixed, although the device may be modified to allow the user to adjustthis distance and to provide an additional mechanism for adjusting thevoltage threshold of the device.

[0066] An added advantage of the embodiment of FIG. 7A is that thedevice may be configured to permit the bellows 221 to expand in responseto increased argon pressure within the reservoir. This will maintain theargon pressure, and thus the threshold voltage of the argon, at a fairlyconstant level despite temperature increases within reservoir 220. Thus,argon plasma will continue to conduct current to the electrodes 132 a132 b etc and ablation may be continued, as it will be a longer periodof time until the threshold voltage of the argon exceeds the voltageapplied by the RF generator.

[0067] An expanding volume embodiment, such as the embodiment of FIG. 7,may be configured such that the volume expands but such that the spacingbetween the internal electrode and the ablation electrode remainsconstant even during expansion of the volume. This allows the system toincrease in volume in response to pressure increases, but permitsconduction between the internal electrode and the ablation electrode fora longer period of time.

[0068]FIGS. 8A through 13B are a series of embodiments that also utilizeargon, but that maintain a fixed reservoir volume for the argon. In eachof these embodiments, current is conducted from an internal electrodewithin the argon reservoir to external ablation electrodes once thevoltage of the internal electrode reaches the threshold voltage of theargon gas.

[0069] Referring to FIGS. 8A and 8B, the fourth embodiment of anablation device utilizes a housing 312 formed of insulating material,overlaying a conductive member 314. Housing 312 includes exposed regions332 in which the insulating material is removed to expose the underlyingconductive member 314. An enclosed reservoir 320 within the housing 212contains argon gas, and an RF electrode member 322 is positioned withinthe reservoir. A return electrode (not shown) is attached to thepatient. The fourth embodiment operates in the manner described withrespect to FIGS. 5A-5D, except that the current returns to the RFgenerator via the return electrode on the patient's body rather than viaone on the device itself

[0070] The fifth embodiment shown in FIGS. 9A and 9B is similar instructure and operation to the fourth embodiment. A conductive member414 is positioned beneath insulated housing 412, and openings in thehousing expose electrode regions 432 of the conductive member 414. Thefifth embodiment differs from the fourth embodiment in that it is abipolar device having a return electrode 430 formed over the insulatedhousing 412. Return electrode 430 is coupled to the RF generator and iscutaway in the same regions in which housing 412 is cutaway; so as toexpose the underlying conductor.

[0071] Internal electrode 422 is disposed within argon gas reservoir420. During use, electrode regions 432 are placed into contact with bodytissue to be ablated. The RF generator is switched on and begins tobuild the voltage of electrode 422 relative to ablation electroderegions 432. As with the previous embodiments, conduction of ablationenergy from electrode 422 to electrode regions 432 will only begin onceelectrode 422 reaches the voltage threshold at which the argon inreservoir 420 ignites to form a plasma. Current passes through thetissue undergoing ablation and to the return electrode 430 on the deviceexterior.

[0072] The sixth embodiment shown in FIG. 10 is similar in structure andoperation to the fifth embodiment, and thus includes a conductive member514, an insulated housing 512 over the conductive member 512 and havingopenings to expose regions 532 of the conductive member. A returnelectrode 530 is formed over the housing 512, and an internal electrode522 is positioned within a reservoir 520 containing a fixed volume ofargon. The sixth embodiment differs from the fifth embodiment in thatthe exposed regions 532 of the conductive member 514 protrude throughthe housing 512 as shown. This is beneficial in that it improves contactbetween the exposed regions 532 and the target body tissue.

[0073] A seventh embodiment is shown in FIGS. 11A through 11C. As withthe sixth embodiment, this embodiment includes an insulated housing 612formed over a conductive member 614, and openings in the insulatedhousing 612 to expose elevated electrode regions 632 of the conductivemember 614. A return electrode 630 is formed over the housing 612. Aninternal electrode 622 is positioned within a reservoir 620 containing afixed volume of argon.

[0074] The seventh embodiment differs from the sixth embodiment in thatthere is an annular gap 633 between the insulated housing 614 and theelevated regions 632 of the conductive member 614. Annular gap 633 isfluidly coupled to a source of suction and/or to an irrigation supply.During use, suction may be applied via gap 633 to remove ablationbyproducts (e.g. tissue and other debris) and/or to improve electrodecontact by drawing tissue into the annular regions between electroderegions 632 and ground electrode 630. An irrigation gas or fluid mayalso be introduced via gap 633 during use so as to flush ablationbyproducts from the device and to cool the ablation tip and the bodytissue. Conductive or non-conductive fluid may be utilized periodicallyduring the ablation procedure to flush the system.

[0075] Annular gap 633 may also be used to deliver argon gas intocontact with the electrodes 632. When the voltage of the electroderegions 632 reaches the threshold of argon delivered through the gap633, the resulting argon plasma will conduct from electrode regions 632to the ground electrode 630, causing lateral sparking between theelectrodes 632, 630. The resulting sparks create an “electrical file”which cuts the surrounding body tissue

[0076] An eighth embodiment of an ablation device is shown in FIGS. 12Band 12C. This device 710 is similar to the device of the fifthembodiment, FIGS. 9A and 9B, in a number of ways. In particular, device710 includes a conductive member 714 positioned beneath insulatedhousing 712, and openings in the housing which expose electrode regions732 of the conductive member 714. A return electrode 730 is formed overthe insulated housing 712. Internal electrode 722 is disposed within anargon gas reservoir 720 having a fixed volume.

[0077] The eighth embodiment additionally includes a pair of telescopingtubular jackets 740, 742. Inner jacket 740 has a lower insulatingsurface 744 and an upper conductive surface 746 that serves as a secondreturn electrode. Inner jacket 740 is longitudinally slidable betweenproximal position 740A and distal position 740B.

[0078] Outer jacket 742 is formed of insulating material and is slidablelongitudinally between position 742A and distal position 742B.

[0079] A first annular gap 748 is formed beneath inner jacket 740 and asecond annular gap 750 is formed between the inner and outer jackets740, 742. These gaps may be used to deliver suction or irrigation to theablation site to remove ablation byproducts.

[0080] The eighth embodiment may be used in a variety of ways. As afirst example, jackets 740, 742 may be moved distally to expose lessthan all of tip electrode assembly (i.e. the region at which theconductive regions 732 are located). This allows the user to expose onlyenough of the conductive regions 732 as is needed to cover the area tobe ablated within the body. Secondly, in the event bleeding occurs atthe ablation site, return electrode surface 730 may be used as a largesurface area coagulation electrode, with return electrode surface 746serving as the return electrode, so as to coagulate the tissue and tothus stop the bleeding. Outer jacket 742 may be moved proximally ordistally to increase or decrease the surface area of electrode 746.Moving it proximally has the effect of reducing the energy density atthe return electrode 746, allowing power to be increased to carry outthe coagulation without increasing thermal treatment effects at returnelectrode 746.

[0081] Alternatively, in the event coagulation and/or is needed,electrode 730 may be used for surface coagulation in combination with areturn patch placed into contact with the patient.

[0082] FIGS. 13A-13B show a ninth embodiment of an ablation deviceutilizing principles of the present invention. The ninth embodimentincludes an insulated housing 812 having an argon gas reservoir 820 offixed volume. A plurality of ablation electrodes 832 are embedded in thewalls of the housing 812 such that they are exposed to the argon inreservoir 832 and exposed on the exterior of the device for contact withbody tissue. A return electrode 830 is formed over the housing 812, butincludes openings through which the electrodes 832 extend. An annulargap 833 lies between return electrode 830 and housing 812. As withprevious embodiments, suction and/or irrigation may be provided throughthe gap 833. Additionally, argon gas may be introduced through theannular gap 833 and into contact with the electrodes 832 and body tissueso as to allow argon gas ablation to be performed.

[0083] An internal electrode 822 is positioned within reservoir 820.Electrode 822 is asymmetrical in shape, having a curved surface 822 aforming an arc of a circle and a pair of straight surfaces 822 b formingradii of the circle. As a result of its shape, the curved surface of theelectrode 820 is always closer to the electrodes 832 than the straightsurfaces. Naturally, other shapes that achieve this effect mayalternatively be utilized.

[0084] Electrode 822 is rotatable about a longitudinal axis and can alsobe moved longitudinally as indicated by arrows in FIGS. 13A and 13B.Rotation and longitudinal movement can be carried out simultaneously orseparately. This allows the user to selectively position the surface 822a in proximity to a select group of the electrodes 832. For example,referring to FIGS. 13A and 13B, when electrode 822 is positioned asshown, curved surface 822 a is near electrodes 832 a, whereas no part ofthe electrode 822 is close to the other groups of electrodes 832 b-d.

[0085] As discussed earlier, the voltage threshold required to causeconduction between internal electrode 822 and ablation electrodes 832will decrease with a decrease in distance between the electrodes. Thus,there will be a lower threshold voltage between electrode 822 and theablation electrodes (e.g. electrode 832 a) adjacent to surface 822 athan there is between the electrode 822 and ablation electrodes that arefarther away (e.g. electrodes 832 b-d). The dimensions of the electrode822 and the voltage applied to electrode 822 are such that a plasma canonly be established between the surface 822 a and the electrodes it isclose to. Thus, for example, when surface 822 a is adjacent toelectrodes 832 a as shown in the drawings, the voltage threshold betweenthe electrodes 822 a and 832 a is low enough that the voltage applied toelectrode 822 will cause plasma conduction to electrodes 832 a. However,the threshold between electrode 822 and the other electrodes 832 b-dwill remain above the voltage applied to electrode 822, and so therewill be no conduction to those electrodes.

[0086] This embodiment thus allows the user to selectively ablateregions of tissue by positioning the electrode surface 822 a close toelectrodes in contact with the regions at which ablation is desired.

[0087]FIG. 14A shows a tenth embodiment of an ablation device utilizingvoltage threshold principles. The tenth embodiment includes a housing912 having a sealed distal end containing argon. Ablation electrodes 932a-c are positioned on the exterior of the housing 912. An internalelectrode 922 is disposed in the sealed distal end. Positioned betweenthe internal electrode 922 and the electrodes 932 a-c is a conductivegrid 933.

[0088] When electrode 922 is energized, there will be no conduction fromelectrode 922 to electrodes 932 a-c until the potential betweenelectrode 922 and the body tissue/fluid in contact with electrodes 932a-c reaches an initiating threshold voltage at which the argon gas willform a conductive plasma. The exact initiating threshold voltage isdependent on the argon pressure, its flowrale (if it is circulatingwithin the device), and the distance between electrode 922 and thetissue/body fluid in contact with the ablation electrodes 932 a-c.

[0089] Because the RF generator voltage output varies sinusoidally withtime, there are phases along the RF generator output cycle at which theRF generator voltage will drop below the voltage threshold. However,once the plasma has been ignited, the presence of energized plasma ionsin the argon will maintain conduction even after the potential betweenelectrode 922 and the body fluid/tissue has been fallen below theinitiating threshold voltage. In other words, there is a thresholdsustaining voltage that is below the initiating threshold voltage, butthat will sustain plasma conduction.

[0090] In the embodiment of FIG. 14A, the grid 933 is spaced from theelectrodes 932 a-c by a distance at which the corresponding plasmaignition threshold is a suitable ablation voltage for the application towhich the ablation device is to be used. Moreover, the electrode 922 ispositioned such that once the plasma is ignited, grid 933 may bedeactivated and electrode 922 will continue to maintain a potentialequal to or above the sustaining voltage for the plasma. Thus, duringuse, both grid 933 and electrode 922 are initially activated for plasmaformation. Once the potential between grid 933 and body tissue/fluidreaches the threshold voltage and the plasma ignites, grid 933 will bedeactivated. Because ions are present in the plasma at this point,conduction will continue at the sustaining threshold voltage provided byelectrode 922.

[0091] The ability of ionized gas molecules in the argon to sustainconduction even after the potential applied to the internal electrodehas fallen below the initiating threshold voltage can be undesirable. Asdiscussed, an important aspect of voltage threshold ablation is that itallows for high voltage/low current ablation. Using the embodimentsdescribed herein, a voltage considered desirable for the application isselected as the threshold voltage. Because the ablation electrodes areprevented from conducting when the voltage delivered by the RF generatoris below the threshold voltage, there is no conduction to the ablationelectrode during the rise time from 0 V to the voltage threshold. Thus,there is no resistive heating of the tissue during the period in whichthe RF generator voltage is rising towards the threshold voltage.

[0092] Under ideal circumstances, conduction would discontinue duringthe periods in which the RF generator voltage is below the threshold.However, since ionized gas remains in the argon reservoir, conductioncan continue at voltages below the threshold voltage. Referring to FIG.4A, this results in the sloping trailing edge of the ablation voltagewaveform, which approximates the trailing portion of the sinusoidalwaveform produced by the RF generator (FIG. 3). This low-voltageconduction to the tissue causes resistive heating of the tissue whenonly high voltage ablation is desired.

[0093] The grid embodiment of FIG. 14A may be used to counter the effectof continued conduction so as to minimize collateral damage resultingfrom tissue heating. During use of the grid embodiment, the trailingedge of the ablation voltage waveform is straightened by reversing thepolarity of grid electrode 933 after the RF generator has reached itspeak voltage. This results in formation of a reverse field within theargon, which prevents the plasma flow of ions within the argon gas andthat thus greatly reduces conduction. This steepens the slop of thetrailing edge of the ablation potential waveform, causing a more rapiddrop towards 0 V, such that it approximates the waveform shown in FIG.4B.

[0094]FIGS. 15A and 15B show an eleventh embodiment utilizing principlesof the present invention. As with the tenth embodiment, the eleventhembodiment is advantageous in that it utilizes a mechanism forsteepening the trailing edge of the ablation waveform, thus minimizingconduction during periods when the voltage is below the thresholdvoltage. In the eleventh embodiment, this is accomplished by circulatingthe argon gas through the device so as to continuously flush a portionof the ionized gas molecules away from the ablation electrodes.

[0095] The eleventh embodiment includes a housing 1012 having anablation electrodes 1032. An internal electrode 1022 is positionedwithin the housing 1012 and is preferably formed of conductive hypotubehaving insulation 1033 formed over all but the distal-most region. Afluid lumen 1035 is formed in the hypotube and provides the conduitthrough which argon flows into the distal region of housing 1012.Flowing argon exits the housing through the lumen in the housing 1012,as indicated by arrows in FIG. 15A. A pump 1031 drives the argon flowthrough the housing.

[0096] It should be noted that different gases will have differentthreshold voltages when used under identical conditions. Thus, duringuse of the present invention the user may select a gas for the spark gapswitch that will have a desired threshold voltage. A single type of gas(e.g. argon) may be circulated through the system, or a plurality ofgases may be mixed by a mixer pump 103 la as shown in FIG. 15C, forcirculation through the system. Mixing of gases is desirable in that itallows a gas mixture to be created that has a threshold voltagecorresponding to the desired treatment voltage. In all of the systemsusing circulated gas, gas leaving the system may be recycled through,and/or exhausted from, the system after it makes a pass through thespark gap switch.

[0097]FIGS. 16A through 16D schematically illustrate the effect ofcirculating the argon gas through the device. Circulation preferably iscarried out at a rate of approximately 0.1 liters/minute to 0.8liters/minute.

[0098] Referring to FIG. 16A, during initial activation of the RFgenerator, the potential between internal electrode 1022 and ablationelectrode 1032 is insufficient to create an argon plasma. Argonmolecules are thus non-ionized, and the voltage measured at the load Lis 0V. There is no conduction from electrode 1022 to electrode 1032 atthis time.

[0099]FIG. 16B shows the load voltage measured from internal electrode1022 across the body fluid/tissue to return electrode 1030. Once the RFgenerator voltage output reaches voltage threshold V_(T) of the argon,argon molecules are ionized to create a plasma. A stream of the ionizedmolecules flows from electrode 1022 to electrode 1032 and current isconducted from electrode 1032 to the tissue. Because the argon isflowing, some of the ionized molecules are carried away. Nevertheless,because of the high voltage, the population of ionized molecules isincreasing at this point, and more than compensates for those that flowaway, causing an expanding plasma within the device.

[0100] After the RF generator voltage falls below V_(T), ion generationstops. Ionized molecules within the argon pool flow away as the argon iscirculated, and others of the ions die off. Thus, the plasma beginscollapsing and conduction to the ablation electrodes decreases andeventually stops. The process then repeats as the RF generator voltageapproaches (−V_(T)) during the negative phase of its sinusoidal cycle.

[0101] Circulating the argon minimizes the number of ionized moleculesthat remain in the space between electrode 1022 and electrode 1032. If ahigh population of ionized molecules remained in this region of thedevice, their presence would result in conduction throughout the cycle,and the voltage at the tissue/fluid load L would eventually resemble thesinusoidal output of the RF generator. This continuous conduction at lowvoltages would result in collateral heating of the tissue.

[0102] Naturally, the speed with which ionized molecules are carriedaway increases with increased argon flow rate. For this reason, therewill be more straightening of the trailing edge of the ablation waveformwith higher argon flow rates than with lower argon flow rates. This isillustrated graphically in FIG. 17. The upper waveform shows the RFgenerator output voltage. The center waveform is the voltage outputmeasured across the load (i.e. from the external electrode 1032 acrossthe body tissue/fluid to the return electrode 1030) for a device inwhich the argon gas is slowly circulated. The lower waveform is thevoltage output measured across the load for a device in which the argongas is rapidly circulated. It is evident from the FIG. 17 graphs thatthe sloped trailing edge of the ablation waveform remains when the argonis circulated at a relatively low flow rate, whereas the trailing edgefalls off more steeply when a relatively high flow rate is utilized.This steep trailing edge corresponds to minimized current conductionduring low voltage phases. Flow rates which achieve the maximum benefitof straightening the trailing edge of the waveform are preferable. Itshould be noted that flow rates that are too high can interfere withconduction by flushing too many ionized molecules away during phases ofthe cycle when the output is at the threshold voltage. Optimal flowrates will depend on other physical characteristics of the device, suchas the spark gap distance and electrode arrangement.

[0103] It should also be noted that the distance between internalelectrode 1022 and external electrode 1032 also has an effect on thetrailing edge of the ablation potential waveform. In the graphs of FIG.18, the RF generator output is shown in the upper graph. VP_(PRFG)represents the peak voltage output of the RF generator, V_(T1)represents the voltage threshold of a device having a large separationdistance (e.g. approximately 1 mm) between electrodes 1022 and 1032, andV_(T2) represents the voltage threshold of a device in which electrodes1022, 1032 are closely spaced—by a distance of approximately 0.1 mm. Aspreviously explained, there is a higher voltage threshold in a devicewith a larger separation distance between the electrodes. This isbecause there is a large population of argon molecules between theelectrodes 1022, 1032 that must be stripped of electrons before plasmaconduction will occur. Conversely, when the separation distance betweenelectrodes 1022 and 1032 is small, there is a smaller population ofargon molecules between them, and so less energy is needed to ionize themolecules to create plasma conduction.

[0104] When the RF generator output falls below the threshold voltage,the molecules begin to deionize. When there are fewer ionized moleculesto begin with, as is the case in configurations having a small electrodeseparation distance, the load voltage is more sensitive to thedeionization of molecules, and so the trailing edge of the outputwaveform falls steeply during this phase of the cycle.

[0105] For applications in which a low voltage threshold is desirable,the device may be configured to have a small electrode spacing (e.g. inthe range of 0.001-5 mm, most preferably 0.05-0.5 mm) andnon-circulating argon. As discussed, doing so can produce a load outputwaveform having a steep rising edge and a steep falling edge, both ofwhich are desirable characteristics. If a higher voltage threshold isneeded, circulating the argon in a device with close inter-electrodespacing will increase the voltage threshold by increasing the pressureof the argon. Doing so will yield a highly dense population of chargedions during the phase of the cycle when the RF generator voltage isabove the threshold voltage, but the high flow rate will quickly washmany ions away, causing a steep decline in the output waveform duringthe phases of the cycle when the RF generator voltage is below thethreshold.

[0106] A twelfth embodiment of a system utilizing principles of thepresent invention is shown schematically in FIG. 19. The twelfthembodiment allows the threshold voltage to be adjusted by permitting thespark gap spacing (i.e. the effective spacing between the internalelectrode and the patient contact electrode) to be selected. It utilizesa gas-filled spark gap switch having a plurality of internal electrodes1122 a, 1122 b, 1122 c. Each internal electrode is spaced from patientcontact electrode 1132 by a different distance, D1, D2, D3,respectively. An adjustment switch 1025 allows the user to select whichof the internal electrodes 1122 a, 1122 b, 1122 c to utilize during aprocedure. Since the threshold voltage of a spark gap switch will varywith the distance between the internal electrode and the contactelectrode, the user will select an internal electrode which will set thespark gap switch to have the desired threshold voltage. If a higherthreshold voltage is used, electrode 1022 a will be utilized, so thatthe larger spark gap spacing D1 will give a higher threshold voltage.Conversely, the user will selected electrode 1022 c, with the smallerspark gap spacing, if a lower threshold voltage is needed.

[0107] Several embodiments of voltage threshold ablation systems, andmethods of using them, have been described herein. It should beunderstood that these embodiments are described only by way of exampleand are not intended to limit the scope of the present invention.Modifications to these embodiments may be made without departing fromthe scope of the present invention, and features described in connectionwith some of the embodiments may be combined with features described inothers of the embodiments. It is intended that the scope of theinvention is to be construed by the language of the appended claims,rather than by the details of the disclosed embodiments.

[0108] Any and all patents, patent applications and printed publicationsreferred to above are incorporated by reference.

What is Claimed is:
 1. An electrosurgical device, for use with anoscillating electrical generator, for treating target tissue comprising:a proximal portion; a distal portion comprising: an insulated housinghaving an outer surface and an opening impervious to liquid and solidmaterial formed therein; and a treatment electrode, the treatmentelectrode comprising an outer electrode surface exposed through theopening and positioned inwardly of the outer surface of the housing,whereby during use a gap is created between the outer electrode surfaceand target tissue so that target tissue does not touch the outerelectrode surface; an electrical pathway, along at least the proximaland distal portions, electrically coupleable to the generator andextending at least through the gap; and the electrical pathwaycomprising a voltage threshold switch, the switch having a thresholdvoltage to prevent conduction of current along the electrical pathwayand to the treatment electrode until the voltage across the switchreaches a threshold voltage.
 2. The device according to claim 1 whereinthe insulated housing comprises a plurality of said openings and thetreatment electrode comprises a plurality of said outer electrodesurfaces.
 3. The device according to claim 1 wherein the treatmentelectrode comprises a hollow member surrounded by the housing.
 4. Thedevice according to claim 3 wherein the insulated housing comprises aplurality of openings formed therein with a plurality of the regions ofthe treatment electrode exposed through the plurality of openings. 5.The device according to claim 1 further comprising means for preventinga liquid or tissue from contacting the outer electrode surface duringuse.
 6. The device according to claim 1 wherein the voltage thresholdswitch prevents said conduction on a cycle-to-cycle basis of theoscillating electrical generator.
 7. The device according to claim 1further comprising a movable insulation member movably positionedrelative to the treatment electrode so to selectively cover a portion ofthe electrode outer surface to affect power transmission through thetreatment electrode.
 8. The device according to claim 7 wherein theinsulation member comprises at least one of the following materials:polyimide, PTFE, glass, ceramic.
 9. The device according to claim 1further comprising a fluid pathway extending along the proximal anddistal portions and to the gap.
 10. The device according to claim 9wherein the fluid pathway is coupled to a suction source so to permitfluid and debris to be removed from the gap.
 11. The device according toclaim 9 wherein the fluid pathway is coupled to a source of gas so topermit gas to be delivered to the gap so that the gas separates theelectrode outer surface from the target tissue.
 12. The device accordingto claim 11 wherein the voltage threshold switch comprises a spark gapswitch created by the electrode and the gas at the gap.
 13. The deviceaccording to claim 9 wherein the fluid pathway is coupled to a source offluid so to permit cooling fluid to be delivered to the gap.
 14. Thedevice according to claim 1 wherein the voltage threshold switchcomprises a spark gap switch.
 15. The device according to claim 14wherein the spark gap switch is located along the distal portion. 16.The device according to claim 14 wherein the spark gap switch comprisesa gas reservoir containing a gas.
 17. The device according to claim 16wherein the gas reservoir is a fixed volume gas reservoir.
 18. Thedevice according to claim 16 wherein the gas reservoir is a variablevolume gas reservoir.
 19. The device according to claim 18 furthercomprising means for automatically expanding the volume of the gasreservoir when the temperature within the gas reservoir increases. 20.The device according to claim 16 further comprising a gas pumping devicefluidly coupled to said gas reservoir so to cause gas in said gasreservoir to flow in the gas reservoir.
 21. The device according toclaim 20 wherein the gas pumping device is capable of pumping gas at arate sufficient to interrupt current flow through the spark gap switchon a cycle-to-cycle basis.
 22. The device according to claim 20 whereinsaid gas pumping device comprises a plurality of gas sources to permitmixtures of different gases to flow into the gas reservoir to therebyadjust the threshold voltage.
 23. The device according to claim 20wherein said gas pumping device is configured to cause the gas in thegas reservoir to circulate therein.
 24. The device according to claim 20wherein said gas pumping device is coupled to a source of gas so tocause gas to flow into and out of said gas reservoir.
 25. The deviceaccording to claim 24 wherein the gas pumping device causes the pressurewithin the gas reservoir to change thereby changing the thresholdvoltage.
 26. The device according to claim 16 wherein the spark gapswitch comprises an intermediate electrode in contact with the gasreservoir.
 27. The device according to claim 26 wherein the treatmentelectrode is in contact with the gas reservoir.
 28. The device accordingto claim 26 further comprising an insulation member positioned relativeto the intermediate electrode so to cover a portion of the intermediateelectrode to affect power transmission through the treatment electrode.29. The device according to claim 28 wherein the insulation member ismovable relative to the intermediate electrode to selectively alter saidpower transmission.
 30. The device according to claim 26 wherein theintermediate electrode is movable within the gas reservoir therebyallowing the threshold voltage to be changed.
 31. The device accordingto claim 26 wherein the intermediate electrode is at a fixed positionwithin the gas reservoir.
 32. The device according to claim 26 whereinthe treatment electrode and the intermediate electrode have exposedsurface areas and are in contact with the gas, said areas sized tocontrol current density between the intermediate electrode and thetreatment electrode.
 33. The device according to claim 26 furthercomprising a periodically electrically energizable element at the gasreservoir, so that a reverse field can be periodically created withinthe gas reservoir to periodically reduce conduction through the switch.34. The device according to claim 16 wherein the gas reservoir isdefined by liquid-impervious structure.
 35. The device according toclaim 16 further comprising means for preventing liquid from enteringthe gas reservoir.
 36. The device according to claim 1 wherein thethreshold voltage switch comprises a first threshold voltage switchwithin the gap between the outer electrode surface and the target tissueand a second threshold voltage switch between the treatment electrodeand the electrical generator, said first threshold voltage switchcomprising a first spark gap switch.
 37. The device according to claim36 wherein: a fluid pathway extends along the proximal and distalportions and to the outer electrode surface; the fluid pathway iscoupled to a source of gas so to permit gas to be delivered to the gapso that the gas separates the outer electrode surface from the targettissue; and the first spark gap switch is created by the other electrodesurface and the gas at the gap.
 38. The device according to claim 36wherein: the second voltage threshold switch comprises a second sparkgap switch; the second spark gap switch comprises a gas reservoircontaining a gas; and the second spark gap switch comprises anintermediate electrode in contact with the gas reservoir.
 39. The deviceaccording to claim 1 further comprising means for cooling said treatmentelectrode.
 40. The device according to claim 1 further comprising meansfor adjusting the current flow through said treatment electrode.
 41. Thedevice according to claim 1 further comprising means for automaticallyadjusting the current flow through said treatment electrode.
 42. Thedevice according to claim 1 further comprising means for adjusting thethreshold voltage.
 43. An electrosurgical device, for use with anoscillating electrical generator, for treating target tissue comprising:a proximal portion; a distal portion comprising an electricallyinsulating tissue-contacting surface and a treatment electrode, thetreatment electrode comprising an electrode outer surface spaced-apartinwardly of the tissue-contacting surface so to create a gap between theelectrode outer surface and target tissue during use; an electricalpathway electrically coupleable to the generator and extending at leastthrough the gap; the electrical pathway comprising a voltage thresholdswitch, the switch having a threshold voltage to prevent conduction ofcurrent along the electrical pathway until the voltage across the switchreaches a threshold voltage; the voltage threshold switch comprising aspark gap switch, the spark gap switch comprising a gas reservoir withinthe distal portion and an intermediate electrode in contact with the gasreservoir; and the distal portion further comprises means for preventingliquid from entering the gas reservoir.
 44. An electrosurgical device,for use with an oscillating electrical generator, for treating targettissue comprising: a proximal portion; a distal portion comprising atreatment electrode and means for preventing the treatment electrodefrom contacting target tissue during use; an electrical pathwayelectrically coupleable to the generator and extending at least throughthe treatment electrode; and the electrical pathway comprising a voltagethreshold switch, the switch having a threshold voltage to preventconduction of current along the electrical pathway until the voltageacross the switch reaches a threshold voltage.
 45. The device accordingto claim 44 wherein the voltage threshold switch comprises a spark gapswitch, the spark gap switch comprising a gas reservoir within thedistal portion and an intermediate electrode in contact with the gasreservoir, and wherein the distal portion further comprises means forpreventing liquid from entering the gas reservoir.
 46. Anelectrosurgical device, for use with an oscillating electrical generatorgenerating an electrical output having a waveform with sloping leadingand trailing waveform edges, for treating target tissue comprising: aproximal portion; a distal portion comprising a treatment electrode andmeans for preventing the treatment electrode from contacting targettissue during use; an electrical pathway, along at least the proximaland distal portions, electrically coupleable to the generator andextending at least through the treatment electrode; the electricalpathway comprising a spark gap type of voltage threshold switch, theswitch comprising a gas reservoir containing a gas and an intermediateelectrode in contact with the gas reservoir; the switch having athreshold voltage to prevent conduction of current through the switchuntil the voltage across the switch reaches a threshold voltage during aleading waveform edge of the electrical output; and means for at leastreducing conduction of current through the switch during at least aportion of a trailing waveform edge of the electrical output.
 47. Thedevice according to claim 46 wherein the current conduction reducingmeans comprises a periodically electrically energizable element at thegas reservoir, so that a reverse field can be periodically createdwithin the gas reservoir to periodically reduce conduction through theswitch.
 48. The device according to claim 47 wherein the electricallyenergizable element is located between the intermediate electrode andthe treatment electrode.
 49. The device according to claim 47 whereinthe electrically energizable element comprises a grid electrode.
 50. Thedevice according to claim 46 wherein the current conduction reducingmeans comprises a gas pumping device fluidly coupled to said gasreservoir so to cause gas in said gas reservoir to flow in the gasreservoir, so that ionized gas molecules within the gas reservoir can becontinuously flushed away.
 51. The device according to claim 50 whereinsaid gas pumping device comprises a plurality of gas sources to permitmixtures of different gases to flow into the gas reservoir.
 52. Thedevice according to claim 50 wherein said gas pumping device isconfigured to cause the gas in the gas reservoir to circulate therein.53. The device according to claim 50 wherein said gas pumping device iscoupled to a source of gas so to cause gas to flow into and out of saidgas reservoir.
 54. An electrosurgical device, for use with anoscillating electrical generator, for treating target tissue comprising:a proximal portion; a distal portion comprising a treatment electrodeand means for preventing the treatment electrode from contacting targettissue during use; an electrical pathway, along at least the proximaland distal portions, electrically coupleable to the generator andextending at least through the treatment electrode; the electricalpathway comprising a spark gap type of voltage threshold switch, theswitch comprising a gas reservoir containing a gas and an intermediateelectrode in contact with the gas reservoir; the switch having athreshold voltage to prevent conduction of current through the switchuntil the voltage across the switch reaches a threshold voltage; and agas moving device fluidly coupled to said gas reservoir so to cause gasin said gas reservoir to flow in the gas reservoir so that ionized gasmolecules within the gas reservoir can be continuously flushed away toprevent conduction through the switch on a cycle-to-cycle basis.